Conventional methods of preparing dehydrated vegetables such as carrots, bell peppers, spinach, cabbage, asparagus, celery, beans, beets, onions, etc. generally involve washing, trimming and/or peeling the vegetables; cutting or slicing or dicing the vegetables; partially precooking the vegetables by blanching them with steam or hot water; treating the vegetables with a preservative; partially drying the vegetables at temperatures of from about 120.degree.F. to about 300.degree.F., generally 130.degree.F. to 250.degree.F., and finish drying the vegetables at lower temperatures until a moisture content of less than about 5 to 10% is reduced. The vegetables so produced have been widely accepted by the food industry as ingredients for use in preparing soups, stocks, casseroles, and the like. However, these dehydrated vegetable products sometimes may be outside of the rigid specifications set by the food processor and consumer regarding microbiological contamination. Relatively high bacteria counts frequently associated with dehydrated vegetables when compared with vegetables preserved by other means such as, for example, freezing, are due in part to the 5-fold to 20-fold concentration effect resulting from the removal of water. In addition, the large-scale processing of a variety of vegetables from many sources in the fields and farms may frequently present serious problems in quality control for a dehydrated vegetable facility. While certain sterilization, preservation and treatment methods for lowering bacteria counts have been and are in use, difficulty is often encountered in carrying out these methods to such an extent as to achieve substantially microbiological-free dehydrated vegetable products without drastically affecting quality, color, flavor, and texture of the finished products. For example, many vegetables are not blanched and many require relatively low temperature, long drying times where conditions for any sort of a "kill" are simply not present. Blanching alters the texture of some vegetables and frequently inhibits drying. High drying temperatures (and faster drying times) are practical for a few products, but many are heat sensitive with color and flavor adversely affected. Chemical treating agents such as sulfite dips and the like may be employed, yet it frequently happens that color, flavor and texture of the dehydrated vegetable products are impaired and residues of such agents may be left which are objectionable thereby making the products wholly unacceptable. Gas sterilization, although effective, is expensive, requires remoistening and redrying and is not approved for vegetables at the present time.
Terminal heat treatment of dry vegetables with steam is a fairly recent and promising development and such method is described and claimed in copending application Ser. No. 426,420, filed Dec. 19, 1973, which is a continuation of Ser. No. 122,133, filed Mar. 8, 1971, now abandoned, which is a continuation of Ser. No. 695,568, filed Jan. 4, 1968, now abandoned in which one of us is also a joint inventor and which is assigned to the assignee of the present application. However, such method requires a redrying step and plant drying capacity may not always be adequate.